Frequency modulation



NOV- 21, i939- LA VERNE R. PHILPoT-r 2,180,369

FREQUENCY MODULATION Filed NOV. l2, 1937 To NEXT STAGE ATTORNEY UNITED STATES `PATENT OFFICE 2,180,369 Y FREQUENCY MonUL'rIoN La. vel-ne n. rmlpoa, washington, n. c. Application November 12, 1937, Serial No. 174,212 13 claims. (o1. 17a-111.5)

(Granted under the act of March 3, 1883, as amended April 30,1928; 370 O. G. 757) This invention relates to a vacuum tube net- Work for obtaining frequency modulation of a carrier of sine wave form and for effecting locked synchronism of one oscillator with the fundamental or a harmonic of another oscillator.

Among the several objects of my invention are:

To provide a simple and effective means for frequency modulating a sine wave carrier .over

wide limits with an unusually high degree of' linearity;

To provide a frequency shift oscillator of high sensitivity;

To provide means for easily adjusting the average frequency of a sine wave frequency modulated carrier by shifting either the value of the tuning elements or by changing the value of a biasing voltage or supply voltage, without impairing the power of the oscillator that is fre-A quency modulated;

To provide means for frequency division;

Toprovide means for non-synchronous frequency control.

The means for accomplishing the foregoing objects together with others that will subsequently appear are shown in the annexedl drawing wherein:

Fig. 1 depicts a vacuum tube network embodying a satisfactory form of my invention;

Fig. 2 shows graphically the relation between frequency and grid voltage under diierent voltage conditions on certain tube elements;

Fig. 3 shows graphically therelation, at .present considered optimum, between plate voltage and suppressor grid voltage;

Fig. 4 illustrates the conditions under which synchronous frequencies may be locked together;

Fig. 5 shows graphically the conditions for non-synchronous frequency control.-

Examination of the static characteristics of a small pentode radio receiving tube such as the type 57 or 6C6 shows that when the suppressor grid is negative with respectl to the cathode potential, negative mutual conductance exists between the screen grid as the anode, and the suppressor grid as the control grid, when the first, or regular, control grid is kept at constant potential. In this condition oscillations can occur in a LCR circuit in the screen grid. lead, provided coupling is established between screen grid and suppressor grid circuits that will swing the suppressor in phase with the screen grid actingv as anode.

Referringto Fig. 1, the tube Ghas an anode 1,

is connected to cathode Il through an impedance I2, lead I3 and resistance I4 which provides the negative bias for the suppressor grid II. Condenser I`5 in parallel withresistance I4 aids in filtering the suppressor grid supply. :screenV grid I0 is connected to -l-B supply lead IB'through impedance I1 and to the high potential side of resistance I4 by variable condenser I8, the impedance I1 and condenser I8 constituting an oscillatory circuit associated with screen grid I0. The screen grid oscillatoryv circuit is coupled to suppressor grid II by condenser I9 to cause'suppressor grid II to swing in phasewith the screen grid I0. Variable bias is impressed upon control grid 9 through variable resistance, or potential divider 20 with filtering by condenser 2| in parallel with the potential divider. Control voltage input is 'by means of transformer 22 whereof the secondary is connected to grids and potential divider 20. The transformer 22 is shown as an audio frequency transformer for purposes of illustrationfsince one application of this invention has been voice modulation of `frequency through a carbon button microphone directly connected to the primary of transformer 22 without amplification between the microphone and the transformer. The network is coupled to the next stage by condenser` 23.

Operation of this network showed that the oscillating frequency was of almost pure sine wave form, as observed on the cathode ray oscilloscope, and by harmonic analysis of the wave .formitselL It was also observed that the frequency of oscillation thereof changed when the voltage on the control grid was varied. I t was' then obvious that if the frequency change ofthe oscillator in response to fluctuating control grid voltages were anything approaching a linear function of the control grid voltage, the appaf ratus could be used as a frequency modulator for a radio frequency carrier. A test of the frequency characteristics was made with I2, I1 and 24 having values of 2.5 millihenries, I9 having a capacity of 250 micromicrofarads and I8 having zero capacity, under the conditions given in the following table:

Plate voltsf Suppressor our screenvolts ,volts quency variation produced by the several control grid voltages given as abscissae in Fig. 2 when the voltages on the screen grid and the suppressor grid had the values given in the table. It will be noted that, in general, the absolute value of the screen grid voltages given in this table are substantially four times the absolute value of the suppressor grid voltages. The optimum relation of screen grid voltages to suppressor grid voltages is shown in the graph of Fig. 3, which corresponds essentially to the relations set forth in the table. It was found that oscillations in the circuit occurred with highest amplitude and persisted over the greatest range of frequency when the voltage relations were asl above dened.

It will be noted that the frequency shift is V considerable for a small change of control grid voltage. Whenthis oscillator was'fed by a carbon button microphone through transformer 22 it was observed that suicient frequency swing was produced to effect satisfactory frequency modulation without amplification between the microphone and the transformer, a maximum shift of i 10 kilocycles being obtained with no observable distortion of quality.

Due to the inherent frequency sensitivity of this circuit, it appeared that it might be used as a locked frequency oscillator. A synchronous frequency was introduced into the control grid circuit, and at very small amplitude immediately locked the two together. Fig, 4 shows this effect for two different set-ups of screen grid and suppressor grid voltage with a bias of two volts on the control grid. The voltages applied to the control grid are given as the abscissae and the frequencies of the control voltages are shown as ordinates. It will 'be noted from the solid line graph in Fig. 4 that one-tenth volt rms signal applied to the control grid could lock in an oscillator of the above type that was within f 25 kilocycles of the applied signal frequency and vthat one-half volt could drag it as far as 100 the control range one of the modulation sidebands took charge, namely the sideband nearest the locked oscillators free frequency and maintained control until the frequency got beyond its range. It has been found that the control voltage may be'introduced in series with impedances I2 and I1 with equally good results except when the circuit is used as a frequency changer.

Another use of this invention is as a non-synchronous frequency control. A control frequency substantially different from the free frequency of the oscillator, but of the same order of magnitude, is impressed on the control grid. Variation of the amplitude of this signal changed the oscillatorl frequency in the direction of the control frequency, as shown in Fig. 5. For example, when the frequency difference was no greater than 20 kilocycles the oscillators locked together at 0.45 volt control. However, this variation of frequency is not linear with applied amplitude and so it couldnot be used for conversion of amplitude modulation to frequency modulation without correction. It is to be noted, however, that in all of the above described uses and tests frequency modulation occurred without change of amplitude of the oscillation.

'I'he frequency of the oscillations iS a funQiOn of applied voltage (other than the control grid voltage) which allows adjusting the free frequency by a `number of means. It also suggests the use of the circuit to control the voltage of a circuit by interpreting properly the frequency of the oscillator and feeding back the output to control the input voltage. For reception of frequency modulated waves, this circuit can be locked with the incoming carrier, to effect a substantial amplication and automatic volume control and static elimination.

For a cathode ray oscillograph driver for visual checking of band pass' characteristics of networks and radio receivers a sine wave voltage (preferably 60 cycles) is applied to the horizontal deilecting plates of thev cathode ray tube and also to the control grid of the oscillator in phase or out of phase. By adjusting the magnitude 0f the swing on the grid, the limits of frequency shift are under control. Changing the value of condenser I8 will adjust the oscillator frequency to the proper point. If the detected output of the network be applied to the vertical deiiecting plates, a graph of the frequency characteristic is drawn on the face of the cathode ray tube with a perfectly linear horizontal frequency scale.

Illustrative of the range of utility of this invention, the following uses to which it has been put may be enumerated:

(a) Voice modulation for radio communication.

(b) Frequency division. Y

(c) Locked synchronous oscillations.

(d) Voltage regulators.

(e) Current regulators.

(f) Reception of frequency modulated carriers.

(g) Band pass driver for cathode ray oscillograph sweep circuit.

(h) Non-synchronous frequency control.

According to the provisions of the patent statutes I have set forth the principle and mode of operation of my invention and have illustrated -and described what I now consider to represent its best embodiment. However, I desire to have it understood that within the scopeof the appended claims the invention may be practiced otherwise than as specifically illustrated and described.

The invention herein described and claimed may be used and/or manufactured by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor,

I claim:

i. A vacuum tube having an anode, a cathode, a control grid, a screen vid. and a suppressor grid; a positive and a negative potential supply lead. means including a highly reactive first inductance connecting said anode to said positive supply lead, a second highly reactive inductance connecting said suppressor grid to said negative supply lead, a flrst resistance connected in series between said cathode and said negative supply lead and to said second inductance whereby the potential drop due to current in said resistance holds said suppressor grid negative with respect to said cathode, means including a third inductance connecting said screen grid to said positive supply lead, a first capacitance tightly coupling said screen grid to said suppressor grid to cause potential variations on said suppressor grid to occur in phase with potential variations on said screen grid, a small second capacita ice connecting said screen grid to said cathode, said third inductance and said second capacitance cooperating with the tube elements respectively connected theretoto constitute a self-oscillatory circuit, an

output circuit connected to said cathode and to a point between said`anode and said rst inductance, means connecting said control grid to said cathode to apply bias potentials to said control grid, and means to apply varying potentials to said control grid to effect frequency modulation of the output of said tube.

2. A vacuum tube having an anode, a cathode, a control grid, a screen grid and a suppressor grid;

' a positive and a negativepotential supply lead,

means including a rst inductance connecting said anode to said positive supply lead, a second inductance connecting said suppressor grid to said cathode, said third inductanceand said second.

capacitance cooperating with the -tube elements respectively connected thereto to constitute a self-oscillatory circuit, means connecting said cathode to said control grid to applyV biasing potentials to said control grid and means to apply varying potentials to said control grid 'to' effect frequency modulation of the output of said tube.

3. A vacuum tube having a cathode, an anode, a control grid, a screen grid and a suppressor grid; a supply lead connected to said anode, a

, resistance connected to said cathode, means including an inductance connecting the side of said resistance remote from said cathode to said suppressor grid to maintain said suppressor grid at a potential lower than the potential of said cathode, a condenser connected across said resistance, means including elements-constituting an oscillatory circuit connecting said screen grid `to the side of said resistance adjacent said cathode and to said anode supply lead, a coupling capressor grid, a capacitance having oneside conpacitance connecting said screen grid to said supnected to the side of said inductance remote from said suppressor grid and its other side connected between said oscillatory circuit and said anode lead, an input circuit including adjustable biasing means connected to said control grid, and an out- -put circuit connected to said anode.

4. A vacuum tube having a cathode, an anode a control grid, a screen grid and a suppressor' grid, means to develop a potential drop by cursol rent through saidtube to maintain said suppressor grid at less than cathode potential, an oscillatory circuit connected to said screen grid and to said cathode, means coupling said suppressor grid l to saidscreen grid to cause said suppressor grid to self-oscillatory relation, which comprises the steps of applying substantially the same value of potential to said screen grid as applied to said anode, maintaining said suppressor grid at a potential less than the potential on said cathode, maintaining an oscillatory condition between said screen grid and said suppressor grid, and apply-l ing modulating potentials to said control grid.

ll 6. The'method of frequency modulation of a radio wave in a system including a vacuum tube having an anode, a cathode, a' control grid, a screen grid, and a suppressorv grid, said screen grid and said suppressor grid being coupled into self-oscillatory relation, which comprises the steps of applying a substantial positive potential to said screen. grid, maintaining said suppressor grid at less than cathode potential, maintaining an oscillatory condition between said screen grid and saidv suppressor grid, and applying modulating potentials to said control grid.

7. Thel method of frequency modulation of a radio wave in a system including a vacuum tube having an anode, a cathode, a control grid, a screen grid, and a suppressor grid, said screen grid and said suppressor grid being coupled into se1f- |`oscillatory relation, which comprises the steps of applying a substantial positive potential to said screen grid, the absolute value of the positive potential on said screen grid being substantially four times the absolute value of the negative potential on said suppressor grid, maintaining said suppressor grid at less than cathode potential, maintaining an oscillatory condition between said screen grid and said suppressor grid, and ap- .y

plyingmodulating potentials to said control grid.

8. A method of operating a space discharge system having a zone wherein electrons are released and spaced therefrom an output zone wherein electrons are absorbed, comprising the potential zone to control the electron stream, a second potential zone wherein the potential is substantially that of said output zone, and a third potential zone wherein the potential is substantially less than that of said zone of release of electrons, effecting self-oscillatory variations of potential in phase in said second and third zones, the absolute value of potential in said second zone being about four times that of the potential in said third zone, and applying in said rst zone varying potentials, whereby the frequency in said output zone is varied as a linear function of said varying potentials in said iirst zone.-

9. A method of operating a space discharge system having a zone wherein electrons are released and spaced therefrom an output zone wherein -`electrons areabsorbed, comprising the stepsl of maintaining between said zones a rst potential zone to control the electron stream, a second potential zone wherein the potential is steps of maintaining between said zones a rst substantially -that of said output zone, and a output zone is varied as a linear function of said l varying potentials in said first zone.

10. A method loi" operating a space discharge system having a zone wherein electrons are released `and spaced therefrom an output zone wherein electrons are absorbed, comprising the steps of maintaining between said zones a first potential'zone to control the electron stream, a I

second potential zone wherein the potential is 1 substantially that of said `output zone, and a third potential zone wherein the potential is substantially less than that of said zone of release of electrons, eiecting self-oscillatory variations o1 potential in phase in said second and third zones, the absolute value of potential in said second zone being about four times that of the potential in said third zone, and applying in said rst zone vary-ing potentials 'of a frequency dif- 11. A vacuum tube having a cathode, an anode,

a control grid, a screen grid and a suppressor grid, means to maintain said suppressor grid at less than cathode potential, a series resonant circuit connected to said screen grid and to said l cathode, means coupling said suppressor grid to said screen grid to cause said suppressor grid to swing in phase with said screen grid, an input circuit connected to said control grid and to said cathode and an output circuit connected to said anode and to said cathode.

12. A vacuum tube having a. cathode, an anode, a control grid, a screen gridand a suppressor grid, means to impress a positive potential on said screen grid, means to impress a negative potential on said suppressor grid, the absolute value of said positive potential being substantially four times the absolute value of said negative potential, a series resonant circuit connected to said screen grid and to said cathode, and means coupling said screen grid to saidsuppressor grid to cause them to swing in phase.

13. A vacuum tube having a cathode, an anode, a' control grid, a screen grid and a suppressor grid, a lead to lsupply positive potential to said anode, a series resonant circuit connected between said lead and said cathode, means connecting said screen grid to said resonant circuit,

means to impress on said suppressor grid a negative potential whereof the absolute value is approximately one-fourth the absolute value of the positive potential on said screen grid, and means `coupling said screen grid and said suppressor grid to cause them to swing in phase.

LA VERNE R. yPHILPO'FJL. 

